The recent development of low loss clad glass fiber by the Corning Glass Company has led to increased interest in the use of such fiber in communications and control systems, particularly those utilizing integrated optics techniques. Such glass fiber is contemplated in many applications to replace copper wire for conduction of signal information. Just as means must be provided for input and output of the electrical signal to copper wire, so also is it necessary to provide optical input and output connections to glass fiber used to conduct signals at optical wavelengths.
One of the advantages of glass fiber over copper cable is its lighter weight. This advantage can be increased if more than one signal path per strand of fiber can be utilized. This is only possible if one can develop a coupling arrangement which provides for a plurality of transmission modes in a single fiber. Duplexing, that is, transmission in both directions on the fiber simultaneously, is also essential to the most economical use of the fiber.